1. Field of the Invention
The present invention relates to an insulated magnetic toner for use in developing an electrostatic image formed by the electrophotographic process, electrostatic recording process, electrostatic printing process, or the like.
2. Description of the Prior Art
At present, the formation of a visible image according to image information, in most cases, is carried out by way of an electrostatic image. This is performed in the manner that an electrostatic image is formed according to a piece of given image information, the electrostatic image is developed by a developer to obtain a toner image, the toner image is transferred usually onto a sheet of copy paper, the transferred toner image is then fixed, whereby a visible image is formed. As the toner for use in developing an electrostatic image in the above method for the image formation a powdery developer is favorably used because of its ease of handling. The powdery developer is classified as the two-component developer and one-component developer, the former being prepared by mixing a carrier composed of iron powder, glass beads, etc., with a toner composed of a binder resin particles containing a coloring agent, and the like, the latter being composed of a magnetic toner alone which consists of binder resin particles containing a powdery magnetic material, and used without being mixed with a carrier. The one-component developer is more desirable than the two-component developer in respect that it is free in itself from the problem lying in the two-component developer that the toner concentration becomes reduced as the toner is used continually.
However, the magnetic toner which has been used as the conventional one-component developer has the disadvantage that it is difficult to obtain high image density and sharpness in the formed visible image as compared to the two-component developer.
The conventional magnetic toner has the disadvantage that its softening point is generally considerably high. This is due to the fact that, even if a binder resin having a low softening point is used in order to incorporate a large amount of a magnetic material into it for the purpose of increasing the density of a visible image, the softening point as of the toner becomes remarkably high. And, as a result, the fixing temperature must be increased, leading possibly to the occurrence of an under-fixing trouble.
The magnetic toner is classified as conductive magnetic toner having a relatively small resistance and insulating magnetic toner having a relatively large resistance.
Incidentally, in the image formation by the xerographic process, there are image quality-affecting important factors: developability and image transferability. The development by the foregoing conductive magnetic toner, because the development progresses in accordance with the electrostatic induction of a latent image charge, has the advantage that the toner need not have a true electric charge and the induction charge value does not change according to humidity, so that the developability is stable, but, on the contrary, has the disadvantage that a turbulence of the line of electric force occurs when a toner image is transferred by electrostatic image transfer means onto a sheet of copy paper, resulting in the "bleeding" of the transferred image. On the other hand, in the development by the insulated magnetic toner, the insulated magnetic toner has a true electric charge of the polarity opposite to that of the latent image charge, and the development progresses as the result of the electric attraction between the true electric charge of the insulated magnetic toner and the latent image charge, and there is the disadvantage that, because the true electric charge of the insulated toner image is always variable according to humidity, the developability varies, whereas there is the advantage that no turbulence of the line of electric force occurs when a toner image is transferred by image transfer means onto a sheet of copy paper, whereby a "bleeding-free" image can be obtained. Thus, the conductive magnetic toner and insulated magnetic toner have reciprocal properties with respect to the relation between the developability and image transferability. It is therefore difficult to obtain consistent quality images with any of the magnetic toners, and this has been a problem awaiting solution.
As means to solve this problem, an attempt was made to prevent the turbulence of the line of electric force at the time of image transfer to thereby improve the image transferability in the manner that, for example, while a conductive magnetic toner is used to develop a latent image, a resincoated paper is used as copy paper. However, such a coated paper, since it requires a resin-coating process in its manufacture, is costly as compared to plain paper, so that it loses the intrinsic merit of the image transfer process that plain paper can be used. Upon this, another attempt has been made to improve the developability by use of an insulated magnetic toner. For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No.31136/1978 describes a method wherein a development is made by charging a toner and applying a bias voltage to the toner. Japanese Patent O.P.I. Publication Nos. 118056/1978 and 22835/1979 describe a method wherein a development is carried out by the use of a toner comprised of a mixture of two magnetic toners different in the resistance. And Japanese Patent O.P.I. Publication No.42141/1979 describes a technique for improving the developability by extremely reducing the thickness of the toner layer in the developing area and also by reducing the interval between the photoreceptor and toner carrier (e.g., a non-magnetic sleeve).
Thus, the above methods have surely improved the developability to a considerable extent even if the insulated magnetic toner is used, but do not yet reach the stage of such developability as obtained by the conductive magnetic toner.
Magnetite and other iron oxides are used as the magnetic material for the conventional magnetic toner. Any of these magnetic materials, because their intensity of saturation magnetization is small, needs to be incorporated in a large quantity into the toner in order to obtain a required magnetic property, such as the intensity of magnetization, for the magnetinc toner. As a result, the conductivity of the magnetic toner is increased, whereas the insulation is lowered, thus resulting in the leakage of the true electric charge of the toner caused by triboelectrically charging; this leakage phenomenon causes the deterioration of developability.
And the large magnetic material content of the toner increases the rate of the magnetic material to appear on the surface of the toner, leading to a large change in the triboelectrically charging characteristic between the individual toner particles or between the toner and other materials. Thus the use of a large quantity of the magnetic material is undesirable because it is disadvantageous for the process. Since magnetite or other iron oxides cause most resins to be negatively charged, this disadvantage becomes conspicuous particularly when obtaining a positively chargeable toner.
The present invention has been made under the above-described background.